Mode selection circuit for a battery, a method of selecting modes for the battery and a battery back-up power supply employing the circuit and method

ABSTRACT

For use with a reserve battery couplable to a charging circuit capable of providing a charging current to the reserve battery, a mode selection circuit and a method of operation thereof. The mode selection circuit includes, in one embodiment, (1) a signal generator that generates a signal based on a temperature of the reserve battery and (2) a mode-changing circuit, coupled to the signal generator, that accepts the signal and selects an alternative one of: (a) a non-charge mode in which the charging current is substantially interrupted when the temperature is greater than a reference temperature and (b) a charge mode in which the charging current is provided to the reserve battery when the temperature is less than the reference temperature.

This Application is a Divisional of prior application Ser. No.08/950,642, filed on Oct. 15, 1997, now U.S. Pat. No. 6,037,747. Theabove-listed Application is commonly assigned with the present inventionand is incorporated herein by reference as if reproduced herein in itsentirety.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to power supplies and,more specifically, to a mode selection circuit for a battery and amethod of operation thereof.

BACKGROUND OF THE INVENTION

In a variety of telecommunications and other applications, batteries[e.g., valve-regulated lead acid (VRLA) batteries] are employed toprovide reserve energy to the equipment powered thereby. With theincreasing trend toward distributed power systems, battery reservesystems are often located in outdoor uncontrolled environments. Over adecade of experience in using VRLA batteries in outdoor environments hasclearly shown that high temperatures drastically reduce the life of thebattery. The lifetime of a typical VRLA battery with a rated life of tenyears at a constant operating ambient temperature of 25° C. will bereduced by a factor of two for approximately every 7° C. rise in averageoperating temperature. When deployed in outdoor environments, thebatteries are generally placed in closed cabinets with poorheat-exchange characteristics. The batteries are, therefore, exposed tohigh temperatures with poor ventilation. As a result, a ten-year ratedbattery may have its lifetime reduced to a quarter or a third of itsrated value, especially in warmer climates such as Dallas, Texas.

While reducing the temperature of the operating environment of thebattery is an important factor in sustaining the life of the battery,there are other ancillary considerations as well. The system employed tomaintain the battery in a state of readiness (i.e., fully charged) isanother important consideration in battery reserve systems. A knowntechnique to improve the life of a battery is to employ an intermittentcharging system. An intermittent charging system is disclosed in A NewConcept: Intermittent Charging of Lead Acid Batteries inTelecommunication Systems, by D. P. Reid, et al. (Reid), Proceedings ofINTELEC 1984, pp. 67-71, which is incorporated herein by reference.

Since the commercial AC power source is typically available about 99.9%of the time, the battery is conventionally maintained in a float modewhereby the battery is fully charged and is essentially being topped-offcontinuously. With an intermittent charging system, the battery is onlycharged a fraction of the time and, otherwise, the battery isdisconnected from the charging circuit. Such a system is very sensiblewith VRLA batteries especially in view of the fact that VRLA batteriessuffer from relatively low self-discharge rates (e.g., less than 10%over a 180 day period at about 25° C.). Analogous to the loss of batterycapacity at higher temperatures, it is estimated that the self-dischargerate approximately doubles for every 10° C. rise in temperature. Evenwith the increase in self-discharge rates associated with higheroperating temperatures, a relatively low duty cycle (i.e., ratio of thecharging time to total time) is sufficient to maintain the battery in astate of readiness should the commercial power source be interrupted.

Since elements of the battery experience aging during float charging(e.g., excess current contributes to grid corrosion of the positiveplate of the battery and water loss), it would be advantageous todecrease the period of time that the battery is in the float mode. Asdisclosed in Reid, the life of a battery may double by employing a 50%float duty cycle over a full float duty cycle operation for a particularbattery design. Therefore, a reduction in the float mode duty cyclesignificantly increases the life of the battery.

As previously mentioned, Reid discloses a system that intermittentlycharges the battery. Reid fails to recognize, however, that floatcharging contributes to excess current charging of the battery (inexcess of the charge necessary to compensate for the charge beingreplenished or lost during self-discharge) thereby unnecessarily heatingthe battery. The reactions that diminish battery life during floatcharging are accelerated at higher temperatures thereby furthercontributing to the degradation of the life of the battery.

Accordingly, what is needed in the art is a recognition that charging abattery under particular environmental conditions affects the life ofthe battery and, more particularly, what is needed is an intermittentsystem for charging a battery that overcomes the deficiencies in theprior art.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides for use with a reserve battery couplable to acharging circuit capable of providing a charging current to the reservebattery, a mode selection circuit and a method of operation thereof.

The mode selection circuit includes, in one embodiment, (1) a signalgenerator that generates a signal based on a temperature of the reservebattery and (2) a mode-changing circuit, coupled to the signalgenerator, that accepts the signal and selects an alternative one of:(a) a non-charge mode in which the charging current is substantiallyinterrupted when the temperature is greater than a reference temperatureand (b) a charge mode in which the charging current is provided to thereserve battery when the temperature is less than the referencetemperature.

The present invention, therefore, recognizes that float charging abattery at higher ambient temperature conditions has deleterious effectson the life of the battery. By charging the battery, in one embodiment,during the periods of time when the ambient temperatures are lower andproviding substantially zero charging current to the battery otherwise,additional improvements in extending battery life are possible.Consequently, the mode selection circuit alternatively selects amutually-exclusive one of the non-charge or charge mode for a batteryreserve system. The intermittent charging system provides an effectivetechnique for protecting and extending the life of the reserve battery.

The foregoing has outlined, rather broadly, a preferred and alternativefeature of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features of the invention will be describedhereinafter that form the subject of the claims of the invention. Thoseskilled in the art appreciate that they can readily use the disclosedconception and specific embodiment as a basis for designing or modifyingother structures for carrying out the same purposes of the presentinvention. Those skilled in the art also realize that such equivalentconstructions do not depart from the spirit and scope of the inventionin its broadest form.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a schematic diagram of a battery back-up power supplyproviding an environment for the present invention;

FIG. 2 illustrates a schematic diagram of another battery back-up powersupply providing an environment for the present invention;

FIG. 3 illustrates a schematic diagram of a battery back-up power supplyincorporating an embodiment of a mode selection circuit constructedaccording to the principles of the present invention;

FIG. 4 illustrates a diagram of a temperature verses time profile for anexemplary reserve battery in a typical outdoor application;

FIG. 5 illustrates a schematic diagram of a battery back-up power supplyincorporating another embodiment of a mode selection circuit constructedaccording to the principles of the present invention; and

FIG. 6 illustrates a schematic diagram of a battery back-up power supplyincorporating another embodiment of a mode selection circuit constructedaccording to the principles of the present invention.

DETAILED DESCRIPTION

Referring initially to FIG. 1, illustrated is a schematic diagram of abattery back-up power supply 100 providing an environment for thepresent invention. The power supply 100 includes a source of electricalpower 110 coupled to an AC/DC rectifier 120. The AC/DC rectifier 120 isthen couplable to a reserve battery 130 and is adapted to float chargethe reserve battery 130. The power supply also includes a disconnectswitch 140 coupled to the reserve battery 130. When the reserve battery130 is almost completely discharged following the loss of AC power, thedisconnect switch 140 decouples the reserve battery 130 from a load 150to avoid putting the reserve battery 130 into a deep discharge. Thepower supply 100 provides power to the load 150.

Turning now to FIG. 2, illustrated is a schematic diagram of anotherbattery back-up power supply 200 providing an environment for thepresent invention. The power supply 200 includes a source of electricalpower 210 coupled to an AC/DC rectifier 220. The AC/DC rectifier 220 isthen couplable to a first reserve battery 230 and a second reservebattery 250 and is adapted to float charge both batteries. The powersupply 200 also includes a first disconnect switch 240 coupled to thefirst reserve battery 230 and a second disconnect switch 260 coupled tothe second reserve battery 250. The disconnect switches 240, 260 connectand disconnect the batteries in an alternating fashion such that atleast one of the batteries is connected at all times. The power supply200 provides power to a load 270.

Power supplies employing multiple battery strings are often employed inwireless applications at lower voltages (e.g., 24 to 48 volts).Obviously, the reserve batteries 230, 250 are limited to charging at aduty cycle of 50% or more. Thus, with n battery strings, the minimumduty cycle is 1/n. The power supply 200, therefore, would benefit froman intermittent charging system as disclosed herein.

Turning now to FIG. 3, illustrated is a schematic diagram of a batteryback-up power supply 300 incorporating an embodiment of a mode selectioncircuit constructed according to the principles of the presentinvention. The power supply 300 includes a source of electrical power310 coupled to an AC/DC rectifier 320 (a charging circuit in theillustrated embodiment). The AC/DC rectifier 320 is coupled to the modeselection circuit that includes a mode-changing circuit, for instance, atemperature-dependant mode-changing circuit, [a parallel coupled switch(e.g., a low-voltage drop metallic contactor 350 and diode 355) and asignal generator, in this embodiment a temperature transducer 360 (e.g.,a thermistor and associated amplifier circuit that generates a voltageproportional to the temperature of the reserve battery 330), which is inturn couplable to a reserve battery 330. The power supply 300 may alsoinclude a disconnect switch 340 coupled to the reserve battery 330. Whenthe reserve battery 330 is almost completely discharged following theloss of AC power, the disconnect switch 340 decouples the reservebattery 330 from a load 380 to avoid putting the reserve battery 330into a deep discharge. The power supply 300 additionally provides powerto the load 380.

The temperature transducer 360 is coupled between the reserve battery330 and the mode-changing circuit. The transducer 360 relays a signal toa control terminal of the switch 350 based upon or indicative of thetemperature of the reserve battery 330. When the sensed temperature ofthe reserve battery 330 drops below a reference temperature, thetransducer 360 signals the switch 350 to close and the reserve battery330 is charged. Furthermore, a charge detection circuit 370 (e.g., avoltage detection circuit that measures the open circuit voltage acrossthe reserve battery 330) may be coupled in parallel with the reservebattery 330 and a control signal is coupled from the charge detectioncircuit 370 to the switch 350. When the battery voltage drops below apredetermined level and the sensed temperature is below the referencetemperature, the switch 350 is closed and the reserve battery 330 ischarged. Conversely, if the battery voltage is above the predeterminedlevel, the switch 350 is opened and a charging current to the reservebattery 330 is substantially interrupted thereby facilitating anon-charging mode of operation.

When the source of electrical power 310 is available and the reservebattery 330 is not being charged, the switch 350 is opened and theoutput voltage of the AC/DC rectifier 320 is adjusted above theopen-circuit voltage of the reserve battery 330 such that the diode 355is reverse biased and no current flows into the reserve battery 330. Thereserve battery 330 experiences a small self-discharge, but since nocurrent is flowing therethrough, no energy from an external sourcedissipates as heat in the reserve battery 330 thereby extending the lifeof the reserve battery 330.

In the event of a failure of the source of electrical power 310, theoutput voltage of the AC/DC rectifier 320 drops to a point where thediode 355 begins conducting and the reserve battery 330 then immediatelypowers the load 380. The switch 350 closes soon after the diode 355begins conducting such that the reserve battery 330 can deliver thepower to the load 380. Of course, the switch 350 and diode 355 mayinclude any type of switching device and still be within the broad scopeof the present invention.

Again, to charge the reserve battery 330 when the source of electricalpower 310 is available and the temperature is in the proper region, theswitch 350 is closed and the voltage of the AC/DC rectifier 320 isadjusted to float-charge the reserve battery 330. To stop charging thereserve battery 330, the switch 350 is opened and the voltage of theAC/DC rectifier 320 is maintained above the open-circuit voltage of thereserve battery 330 such that the diode 355 is in a nonconducting state.In this manner, the reserve battery 330 can be intermittently charged(even with only a single battery string) without limiting the immediateavailability of the reserve battery 330 to power the load 380. Theintermittent charging system can, therefore, provide any charging dutycycle even with multiple battery strings in parallel.

Turning now to FIG. 4, illustrated is a diagram of a temperature versestime profile for an exemplary reserve battery in a typical outdoorapplication. As time cycles from night to day to night, the reservebattery will be charged during the time when the transducer senses atemperature below a reference level. In one embodiment, when the batterytemperature is above the reference level, the reserve battery will notbe charged. As illustrated in the profile, one methodology to implementbattery charging is to charge the battery during the night when thetemperature of the battery plant is lower. While many alternativeschemes are available, it is advantageous to charge the battery when theenvironmental temperature is below a reference temperature (e.g., 30°C.) to, ultimately, extend the life of the battery. Those skilled in theart understand that any type of battery such as a valve-regulated leadacid (VRLA) battery is well within the broad scope of the presentinvention. Those skilled in the art also understand that the referencetemperature includes any temperature (depending on, for instance, theclimate of the installation) and the temperature may be periodically orcontinually modified (depending on, for instance, the working conditionsof the installation) and still be within the broad scope of the presentinvention.

Turning now to FIG. 5, illustrated is a schematic diagram of a batteryback-up power supply 500 incorporating another embodiment of a modeselection circuit constructed according to the principles of the presentinvention. The power supply 500 includes a source of electrical power510 coupled to an AC/DC rectifier 520. The AC/DC rectifier 520 is thencoupled to the mode selection circuit that includes a mode-changingcircuit (a low voltage disconnect circuit including a controlledswitching device 550, e.g., an isolated-gate bipolar transistor, and adisconnect switch 540) and a signal generator (e.g., a temperaturetransducer 560), which is in turn couplable to a reserve battery 530.When the reserve battery 530 is almost completely discharged followingthe loss of AC power, the disconnect switch 540 decouples the reservebattery 530 from a load 580 to avoid putting the reserve battery 530into a deep discharge. Additionally, the power supply 500 provides powerto the load 580.

The temperature transducer 560 is coupled between the reserve battery530 and the low voltage disconnect circuit. The transducer 560 relays acontrol signal to the disconnect switch 540 based upon or indicative ofthe temperature of the reserve battery 530. When the sensed temperatureof the reserve battery 530 drops below a reference temperature, thetransducer 560, sensing the drop, signals the disconnect switch 540 toclose and the reserve battery 530 is charged. Furthermore, a chargedetection circuit 570 is coupled in parallel with the reserve battery530 and a control signal is coupled from the charge detection circuit570 to the low voltage disconnect circuit. When the voltage of thereserve battery 530 drops below a predetermined level and the sensedtemperature is below the reference temperature, the disconnect switch540 is closed and the reserve battery 530 is charged. Conversely, if thebattery voltage is above the predetermined level, the disconnect switch540 is opened and a charging current to the reserve battery 530 issubstantially interrupted thereby facilitating a non-charging mode ofoperation.

When the reserve battery 530 is not being charged, the disconnect switch540 and the controlled switching device 550 are both open. In the eventof a loss of AC power, the disconnect switch 540 and the controlledswitching device 550 are both transitioned on. The controlled switchingdevice 550 transitions on rapidly (within a few microseconds) ensuringthat the power to the load 580 is not disrupted. The disconnect switch540 turns on a few milliseconds later and due to its lower on-state dropreduces the dissipation in the controlled switching device 550 to nearzero. Thereafter, the controlled switching device 550 may be turned off.Since the controlled switching device 550 carries current for only a fewmilliseconds, the energy dissipation therethrough is relatively small.If the voltage of the reserve battery 530 falls below a predeterminedthreshold, the disconnect switch 540 may be opened to disconnect thereserve battery 540 from the load 580. Of course, any controllableswitching device may be employed in lieu of the switching devicesillustrated and described above.

Turning now to FIG. 6, illustrated is a schematic diagram of a batteryback-up power supply 600 incorporating another embodiment of a modeselection circuit constructed according to the principles of the presentinvention. The power supply 600 includes a source of electrical power610 coupled to an AC/DC rectifier 620. The AC/DC rectifier 620 is thencoupled to a reserve battery 630. The power supply 600 also includes adisconnect switch 640 coupled to the reserve battery 630. Additionally,the power supply 600 provides power to a load 680.

The illustrated embodiment employs a combination of charging current andnon-charging current to provide intermittent charging. By monitoring thecurrent of the reserve battery 630 and using a current control loop 650,the charging of the reserve battery 630 occurs as follows. The reservebattery 630 is float charged by regulating the float voltage. When thecharging of the reserve battery 630 is to be terminated, the outputvoltage of the AC/DC rectifier 620 is adjusted by the current controlloop 650 to a value near the open-circuit voltage of the reserve battery630 thereby controlling the current therethrough within a small value.

More specifically, a clock 660 (or any timing device) is coupled to thecurrent control loop 650 (forming part of the mode selection circuit).The clock 660 relays a signal to the current control loop 650, that maybe based upon or indicative of a temperature of the reserve battery 630.At a predetermined time period, a current sufficient to charge thereserve battery (a charging current) is provided to the reserve battery630 during a charge mode. At a time period other than the predeterminedtime period, the charging current is substantially interrupted to thereserve battery 630 during a non-charge mode. In the event of a loss ofthe source of electrical power 610, the reserve battery 630 is alreadyconnected to the load so no additional steps are necessary. Thisparticular embodiment does not require any additional connections exceptmonitoring the current into the reserve battery 630 and the control ofthe charging current or voltage. Of course, other techniques to controlthe current through the reserve battery 630 are well within the broadscope of the present invention.

The above noted descriptions include possible embodiments of the presentinvention. Other possible variations include, without limitation:

Monitoring battery temperature and estimating the self discharge rateswhen the battery is not being charged, and when the estimated totalself-discharge of the battery has dipped below a threshold, charge thebattery to replenish the charge;

Varying the charging time and monitoring the charging current toreplenish the charge estimated to have been lost due to self discharge(with some excess charging to account for errors in estimating the selfdischarge), so as to further minimize overcharging; and

When the battery is "not being charged", maintain the battery voltage ata value (probably temperature dependent) that maximizes the life of thebattery.

Thus, in the alternative embodiment of the present invention, theintermittent charging system is related to a time of day or period oftime. In conjunction with historical weather trends, for instance, thebattery may be charged during specific times of the day when thetemperature is at its coolest. For instance, the battery may beautomatically charged from 2:00 to 4:00 AM everyday or periodically,depending on the expected discharge rate of the battery. Those skilledin the art understand that various techniques to intermittently chargethe battery in connection with predetermined time periods are availableand are well within the broad scope of the present invention.

Although the present invention has been described in detail, thoseskilled in the art understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the invention in its broadest form.

What is claimed is:
 1. For use with a reserve battery couplable to acharging circuit capable of providing a charging current to said reservebattery, a mode selection circuit, comprising:a signal generator thatgenerates a signal based on a time of day; and a mode-changing circuit,coupled to said signal generator, that accepts said signal and selectsan alternative one of:a charge mode in which said charging current isprovided to said reserve battery during a predetermined time of day, anda non-charge mode in which said charging current is substantiallyinterrupted during time periods other than said predetermined time ofday.
 2. The circuit as recited in claim 1 further comprising a chargedetection circuit, coupled to said reserve battery, that monitors acharge of said reserve battery, said mode-changing circuit furtherselecting an alternative one of:said charge mode in which said chargingcurrent is provided to said reserve battery during said predeterminedtime period and when said charge is below said predetermined value, andsaid non-charge mode in which said charging current is substantiallyinterrupted when said charge is above a predetermined value.
 3. Thecircuit as recited in claim 1 wherein said charging circuit comprises anAC/DC rectifier.
 4. The circuit as recited in claim 1 wherein saidreserve battery is a valve-regulated lead acid (VRLA) battery.
 5. Thecircuit as recited in claim 1 wherein said mode-changing circuitcomprises a parallel-coupled switch and diode coupled to said reservebattery.
 6. The circuit as recited in claim 1 wherein said mode-changingcircuit comprises a low-voltage-disconnect (LVD) circuit coupled to saidreserve battery.
 7. The circuit as recited in claim 1 wherein saidmode-changing circuit comprises a current control loop.
 8. The circuitas recited in claim 1 wherein said signal generator comprises a clock.9. For use with a reserve battery couplable to a charging circuitcapable of providing a charging current to said reserve battery, amethod of selecting modes for said reserve battery, comprising:providinga signal based on a time of day with a signal generator; and acceptingsaid signal with a mode-changing circuit, coupled to said signalgenerator, and selecting an alternative one of:a charge mode in whichsaid charging current is provided to said reserve battery during apredetermined time of day, and a non-charge mode in which said chargingcurrent is substantially interrupted during time periods other than saidpredetermined time of day.
 10. The method as recited in claim 9 furthercomprising monitoring a charge of said reserve battery with a chargedetection circuit, coupled to said reserve battery, that method furtherselecting an alternative one of:said charge mode in which said chargingcurrent is provided to said reserve battery during said predeterminedtime period and when said charge is below said predetermined value, andsaid non-charge mode in which said charging current is substantiallyinterrupted when said charge is above a predetermined value.
 11. Themethod as recited in claim 9 wherein said charging circuit comprises anAC/DC rectifier.
 12. The method as recited in claim 9 wherein saidreserve battery is a valve-regulated lead acid (VRLA) battery.
 13. Themethod as recited in claim 9 wherein said mode-changing circuitcomprises a parallel-coupled switch and diode coupled to said reservebattery.
 14. The method as recited in claim 9 wherein said mode-changingcircuit comprises a low-voltage-disconnect (LVD) circuit coupled to saidreserve battery.
 15. The method as recited in claim 9 wherein saidmode-changing circuit comprises a current control loop.
 16. The methodas recited in claim 9 wherein said signal generator comprises a clock.17. A battery back-up power supply, comprising:a source of electricalpower; an AC/DC rectifier coupled to said source of electrical power; areserve battery couplable to a said AC/DC rectifier, said AC/DCrectifier capable of charging said reserve battery; and a mode selectioncircuit, including:a signal generator that generates a signal based on atime of day, and a mode-changing circuit, coupled to said signalgenerator, that accepts said signal and selects an alternative one of:acharge mode in which said charging current is provided to said reservebattery during a predetermined time of day, and a non-charge mode inwhich said charging current is substantially interrupted during timeperiods other than said predetermined time of day.
 18. The power supplyas recited in claim 17 further comprising a charge detection circuit,coupled to said reserve battery, that monitors a charge of said reservebattery, said mode-changing circuit further selecting an alternative oneof:said charge mode in which said charging current is provided to saidreserve battery during said predetermined time period and when saidcharge is below said predetermined value, and said non-charge mode inwhich said charging current is substantially interrupted when saidcharge is above a predetermined value.
 19. The power supply as recitedin claim 17 wherein said reserve battery is a valve-regulated lead acid(VRLA) battery.
 20. The power supply as recited in claim 17 wherein saidmode-changing circuit comprises a parallel-coupled switch and diodecoupled to said reserve battery.
 21. The power supply as recited inclaim 17 wherein said mode-changing circuit comprises alow-voltage-disconnect (LVD) circuit coupled to said reserve battery.22. The power supply as recited in claim 17 wherein said mode-changingcircuit comprises a current control loop.
 23. The power supply asrecited in claim 17 wherein said signal generator comprises a clock.